(1) Field of the Invention
The present invention generally relates to a Viterbi decoding system used to decode a digital signal in a digital signal processing system, such as a magnetic recording/reproducing apparatus.
(2) Description of the Related Art
In a magnetic recording/reproducing apparatus, such as a magnetic disk apparatus, intersymbol interference of a read signal increases as the recording density of the apparatus increases. A Viterbi decoder, which is based on a maximum-likelihood sequence, is used to decode such a read signal having intersymbol interference.
FIG. 1 is a block diagram of a conventional data transmission system, which includes a convolutional encoder 11, a transmission path 12, a waveform equalizer 13 and a Viterbi decoder 14. The convolutional encoder 11 positioned on the transmitter side convolutionally codes transmission data. The coded data is affected by intersymbol interference while it is transmitted via the transmission path 12. That is, data received by the waveform equalizer 13 has intersymbol interference. The waveform equalizer 13 shapes the waveform of the received data signal. The Viterbi decoder 14 corrects an error in the data signal having the shaped waveform. The Viterbi decoder 14 includes an ACS (Adder/Comparator/Selector) circuit, a path memory and a path selector. These structural elements are based on a constraint length of a convolutional code. The read signal of a magnetic recording/reproducing apparatus, such as a magnetic disk apparatus, has a waveform affected by intersymbol interference like the transmission signal as described above. Hence, it is possible to decode the read signal by the maximum-likelihood process.
FIG.2 is a block diagram of a conventional recording/reproducing apparatus. The apparatus shown in FIG.2 uses a partial-response maximum-likelihood technique. Recorded data is precoded by a precoder 21 and then coded into an NRZI (Non Return Zero Inverse) code by an NRZI encoder 22. The NRZI code is recorded on a magnetic recording/reproducing device 23. The NRZI code is read out from the magnetic recording/reproducing device 23 and is then waveform-equalized by an equalizer 24. The waveform-equalized signal from the equalizer 24 is decoded by a Viterbi decoder 25.
Assuming that a delay time of a bit period of the recorded data is D, an NRZI recording system in which only data "1" is magnetically inverted by the NRZI encoder 22 has a characteristic described by [1/(1-D)]mod2. Further, the characteristic of the magnetic recording/reproducing device 23 is written as (1-D), and the characteristic of the equalizer 24 is written as (1+D). Assuming that the precoder 21 has a characteristic of [1/(1+D)]mod2, a composite characteristic of the precoder 21 and the NRZI recording system is the reverse of a composite characteristic of the magnetic recording/reproducing apparatus 23 and the equalizer 24. Hence, the Viterbi decoder 25 receives the read signal in which it alternately has plus and minus polarities and hence the characteristics of the recording and reproduction are canceled.
FIG.3 is a block diagram of the Viterbi decoder 25 shown in FIG.2. As shown, the Viterbi decoder 25 is composed of an assumed-path memory 31, an ACS circuit 32, a path memory 33 and a path selector 34. The assumed-path memory 31 stores expected values obtained from the waveform of a data train composed of a number of bits corresponding to the constraint length. The ACS circuit 32 includes an adder (A), a comparator (C) and a selector (S). The adder adds, for each of the expected values, a square of the difference between a sampled value of a waveform-equalized read signal from the equalizer 24 and the expected value and a path metric value previously calculated. The comparator compares the added values. The selector selects the smallest one of the added values. The value positioned at the end of a selected assumed path is written into the path memory 33. The value written into the path memory 33 is not a maximum-likelihood value as decoded data, but a likelihood value obtained at the present time. The path selector 34 selects the smallest one of the path metric values obtained at the present time, and selects a path related to the selected smallest path metric value. Data located at the end of the selected path is output as decoded data. The read signal of the magnetic recording/reproducing device 23 has the plus and minus polarities. With the above in mind, each of the assumed-path memory 31 and the path memory 33 is capable of storing three different numerals "-1", "0" and "1".
It is desired that the decoding system composed of the equalizer 24 and the Viterbi decoder 25 shown in FIG.2 be capable of decoding the read signal even if it has a low S/N ratio. The ability of the decoding system is primarily based on increase/decrease in high-frequency noise in the equalizer 24 and a lower limit of the S/N ratio at which the waveform-equalized read signal can be decoded. The above depends on the magnitude of intersymbol interference and the shape of the equalized signal. However, conventional Viterbi decoding systems as described above do not have an arrangement which adjusts the ability of the decoding system in accordance with the magnitude of intersymbol interference so as to maximize ability of the decoding system.